1. Field of the Invention
The present invention relates to an orderwire controller and orderwire control system. More particularly, the present invention relates to an orderwire controller and system which control orderwire voice channels for use by maintenance people.
2. Description of the Related Art
Synchronous Digital Hierarchy (SDH) and Synchronous Optical Network (SONET) are standard specifications which provide core technologies to efficiently multiplex a wide variety of high-speed and low-speed services. In SDH/SONET systems, various control signals for operations and maintenance services are carried by a prescribed section of each transmission frame, called the “overhead.” In particular, the overhead contains E1/E2 bytes for orderwire functions, with which maintenance people working at different network elements can talk to each other.
Normally, the orderwire communication is performed in broadcast mode, allowing one engineer to communicate with two or more other engineers simultaneously. In the case that the network is configured in a ring topology, the orderwire facilities offer ring protection functions to avoid howls, as well as providing ring restoration capabilities to recover from link failure. FIGS. 10 and 11 are diagrams explaining the concept of ring protection functions. More specifically, FIG. 10 shows a situation where orderwire signals propagate over a loop, and FIG. 11 shows how such a loop can be avoided by using ring protection functions.
Referring first to FIG. 10, four network elements (NEs) 101 to 104 are connected in a ring topology. Those NEs 101 to 104 are each equipped with a mixer A1 to A4, which is an analog adder to combine multiple voice signals in analog form. Telephone sets 101-1 to 104-1 are attached to those NEs 101 to 104, respectively, through a transformer (not shown). A digital voice orderwire signal carried on the E1/E2 bytes is converted into an analog voice signal when it has reached an NE. That analog voice signal is then mixed with similar signals sent from other NEs, and the NE sends the resultant signal to its local telephone set 101-1 to 104-1. Each NE also receives an analog voice signal from its local telephone set 101-1 to 104-1. This signal is mixed with similar voice signals sent from other NEs, converted into a digital voice signal, and sent out to the adjacent NEs on the E1/E2 bytes of SONET/SDH frames. In this way, all the telephone sets 101-1 to 104-1 coupled to the NEs 101 to 104 are interconnected by a common orderwire circuit.
With the above-described construct alone, however, the orderwire circuit would make a loop in each ring direction as indicated by the dotted lines in FIG. 10, where the outgoing signal transmitted from one telephone set comes back to itself after a certain propagation delay time. As a result, the speaker would hear the echo of his/her voice through the handset he/she is using, thus suffering an interference known as a howl.
To avoid the problem of howling noises, the NEs 101 to 104 are configured to operate under a mater-slave relationship in which the master station disconnects, or terminates, the orderwire circuit at its east or west side. FIG. 11 illustrates this master-slave configuration. In this example network, the NE 101 works as the master station, while other NEs 102 to 104 as slave stations. The master NE 101 terminates the orderwire circuit at its west ports, breaking the loop of orderwire signals. Now that the NEs 101 to 104 are interconnected in a linear fashion, the howling noise problem will never occur.
Referring next to FIGS. 12 and 13, the concept of ring restoration functions is depicted. More specifically, FIG. 12 shows a situation where a link failure occurs, and FIG. 13 shows how it is resolved by a ring restoration function.
In the system of FIG. 12, the NEs 101 to 104 form a ring network, the master NE 101 terminating its west-side orderwire circuit to avoid howling problems. Suppose here that a link failure occurs as indicated by the dotted circle in FIG. 12. Because of the presence of orderwire termination at the west side of the NE 101, the NEs 101 and 102 cannot communicate with the other NEs 103 and 104 any longer. In such a situation, a link failure would disrupt the delivery of orderwire signals to some member NEs in the ring, as long as the orderwire circuit is terminated at the master NE.
To correct this situation, it is necessary to reconfigure the ring network so that the termination of the orderwire circuit will be removed, and to make this operation possible, the master NE 101 should be notified of the link failure. Referring now to FIG. 13, the NE 103 detects a link communication error and reports it to the master NE 101 via the intermediate NE 104. The master NE 101 then connects the orderwire at its west side, and simultaneously, the NE 103 terminates the orderwire circuit at its west side. The orderwire connection is now recovered from disruption.
While, in the above-described ring system, one network element belongs only to a single ring, there are some other systems where one network element is connected to two or more ring networks. The network element of this type is referred to herein as the “junction node.” FIG. 14 illustrates a system where two ring networks meet at a junction node, NE 100. The NE 100 provides junction functions to support the two rings, ring-a and ring-b. More specifically, the ring-a is formed by four NEs 101 to 103, and the ring-b is formed by four NEs 100 and 201 to 203, the two rings sharing the NE 100 as an intermediary node between them.
Conventionally, the junction node NE 100 provides only one mixer for orderwire signaling, as other normal NEs have. That mixer unit does not support mixing of orderwire signals supplied from a plurality of ring networks. For this reason, the NE 100 should choose either ring-a or ring-b to make orderwire functions operable in such a conventional system. This means that the NE 100 has to disconnect (or terminate) the non-chosen orderwire circuit at both the east and west ports. In the system of FIG. 14, the ring-a orderwire circuit is disconnected as such. This conventional setup of orderwire circuits does not allow the ring-a to form a complete ring network in terms of orderwire signaling, while normal communication data can be transported among the NEs 100 to 103 over the ring-a. (Note that the above termination only applies to orderwire signals, but it does not affect other normal traffic signals.) The ring-b, on the other hand, enables its members to communicate with each other over the orderwire channel, enjoying the benefits of a ring protection and ring restoration functions described earlier. In the illustrated system, the NE 201 serves as the master station of ring-b, the other NEs 100, 202, and 203 being slave stations. The master 201 terminates the orderwire circuit at its west port.
To see what problems are inherent in the above described multi-ring network system, consider the orderwire communication between the NEs 101 and 103 on the ring-a, in comparison with that between the NEs 100 and 203 on the ring-b. Since the orderwire circuit on the ring-b forms a fully functional ring, its ring restoration capabilities are expected to work effectively to save the network from a possible fault. For example, if it encounters a link failure at a portion Pb as shown in FIG. 14, the ring-b restores the orderwire communication through the steps of: (1) detecting the failure Pb at the NE 100, (2) sending a link failure alarm from the slave NE 100 to the master NE 201, and (3) moving the orderwire termination from the west side of the master NE 201 to the east side of the NE 100. In contrast to the ring-b, the ring-a has a problem in its orderwire circuit. More specifically, suppose that a link failure has occurred at a portion Pa shown in FIG. 14. In this case, the NEs 101 and 103 can no longer communicate with each other because their orderwire signals are blocked at the NE 100 in both the east-bound and west-bound directions.
As seen from the above discussion, the conventional junction function implemented in the NE 100 is insufficient in terms of its tolerance to link failures, since it cannot fully support the rings. In other words, the conventional orderwire system has to pose functional limitations on some of the rings. In case of a link failure, the orderwire communication on such rings would be inevitably disrupted, because of the lack of ring restoration functions.
Another problem in the conventional networks is that the NE 100 at the junction point lacks flexibility in the orderwire communication between different rings. The propagation of orderwire signals are confined within a single ring; there is no connectivity between the ring-a and ring-b. Thus, disadvantageously, the conventional networks fail to provide good usability or expandability of a system.
Still another problem in the conventional networks is that they cannot support orderwire communication between A-law systems (used in SDH networks) and Mu-law systems (used in SONET networks). When two ring networks use those different coding methods, they cannot communicate correctly because of the lack of appropriate conversion facilities.